Integrated circuits (ICs) are widely used in many applications. ICs can include a variety of circuitry that can operate as stand alone systems or in conjunction with other circuit components. ICs include an arrangement of pins that provide means for receiving power as well as for communicating information relative to the IC. As a result, ICs are vulnerable to external environmental conditions through such pins. For example, ICs can be damaged by electrostatic discharge (ESD) events, in which large currents flow through the IC usually initiated at one or more pins.
The vulnerability of IC chips to ESD and other events has created need for ESD protection circuits. Consequently, ESD protection circuits are often added to the integral design of IC chips. Many conventional ESD protection schemes for ICs employ peripheral dedicated circuits to carry the ESD currents from the pin or pad of the device to ground by providing a low impedance path. In this way, the ESD currents flow through the ESD protection circuitry, rather than through the more vulnerable circuits in the chip.
FIG. 1 depicts an example of part of an IC 2 that includes an electrically erasable programmable read-only memory (EEPROM) 4. The EEPROM 4 can be programmed by providing an enable input signal at a pin (ENB) 6. The pin 6 also can provide an enable signal to other circuitry, such as to enable internal logic. One or more ESD protection circuits 8 can be associated with the pin 6. For example, a first ESD protection circuit can be connected directly at the pin 6. Another ESD protection circuit 8 can be connected to the signal path between a resistor R and the EEPROM 4. The ESD protection circuit 8, for example, can include a fast switch or clamp and RC filter that are activated as a function of the signal at the pin 6 to shunt current away from the EEPROM 4 during an ESD event. The ESD protection circuit 8 can divert current from the input and discharge the current through the ESD protection circuit, thus protecting the EEPROM 4 from damage resulting from the ESD event.
Conventional ESD protection, however, may not be capable of providing adequate protection in all circumstances. For instance, some types of ICs include programmable circuitry, such as electrically programmable read only memory (EPROM), that may require higher voltages be applied at one or more pins to implement certain modes (e.g., programming mode) of operation. Since the higher voltages are applied to the ICs, conventional ESD protection have to be modified to allow operation at the higher voltages and thus may become ineffective in suppressing certain transient events. Accordingly, a spike in the input voltage at an input pin, such as may be due to applying an external voltage or to parasitic conditions, could cause inadvertent re-programming or poor field retention by certain IC chips.